1. Field of the Invention
The invention relates to the extension of a metallic pair connection via a digital transport path.
2. Description of the Related Art
The traditional telecommunication system was comprised of a twisted pair of copper wires which ran from the central office of a telephone company at a particular location to the individual subscriber's home or office. With the advent of digital transmission systems, such as fiber optic networks, the central offices were themselves connected by fiber optic cables, and later, digitally coupled to remote terminals which were placed closer to subscriber drops so that the higher bandwidth digital transmission system could be extended closer to the subscriber's home. More recently, the wire pairs between the central office and remote terminals are being replaced by high bandwidth fiber optic transmission systems. The metallic pair connections are still the dominant means of interconnecting a subscriber's home or office with the remote terminal and its digital link to the central office.
Under certain circumstances, it is necessary to require the digital transmission system to emulate a true metallic pair connection extending between the central office and the remote terminal. One of the most important reasons is to allow test equipment, positioned in the central office, to remotely test subscriber connections extending from the remote terminal.
A number of different schemes have been implemented in order to allow for testing or connection of metallic pairs across the digital transmission system.
FIG. 1A shows a first prior art alternative for testing subscriber lines at the remote end of one type of digital transmission system, a Digital Loop Carrier (DLC) system. As shown in FIG. 1, a typical DLC system includes the Central Office Terminal (COT) and a central office switch at the central office end of the digital transmission system connection. At the remote end of the connection, a digital loop carrier Remote Terminal (RT) is coupled by, for example, a fiber link or high frequency cable to the COT, and to subscriber lines comprising a plurality of metallic pairs (which, for historical reasons, are referred to as "Tip" and "Ring" connections). The central office switch at the CO end is coupled to a pair gain test controller (PGTC) unit, manufactured by Lucent Technologies, Inc. (formerly AT&T Technologies, Inc.), a well-known apparatus for testing the customer's cable pair extension or drop beyond the transmission system's remote terminal (RT).
The PGTC acts as an interface between the DLC systems and loop testing systems. The PGTC allows the use of conventional cable pair loop testing methods on the subscriber lines beyond the remote terminal, provides for testing of a customer's carrier channel equipment, including both transmission performance and signalling performance, and provides compatibility with automated test systems, such as the Mechanized Loop Testing (MLT) system (also manufactured by Lucent Technologies, Inc.). The PGTC therefore minimizes the loop testing equipment needed per carrier system. In operation, the PGTC will be coupled to a test system such as MLT, which is itself controlled by, for example, a Loop Maintenance Operations System (LMOS).
In FIG. 1A, a physical metallic bypass pair 12 is coupled between the central office terminal and remote terminal. The metallic bypass pair is coupled through the central office terminal to the tester trunk of the PGTC unit, and may be switched at the central office and remote terminal to connect with any of the subscriber lines at the remote unit. The advantage of the metallic bypass pair is that no additional test equipment, beyond the PGTC, is required to conduct testing at the remote end. However, the metallic bypass pair is not available with the fiber optic plant; that is, a separate line must be run from the COT to the RT. Secondly, when using the PGTC, one is limited to testing of "Plain Old Telephone Service" (POTS) lines only.
Other types of "special" service lines may also be provided by the telecommunication system. Such special services may comprise, for example, special rate tariff lines, data lines, and PBX lines. Test systems used for special services testing are different from the POTS test systems. The tests applied to special services circuits are more extensive and have separate databases from those used for POTS testing.
A second alternative to remote testing of subscriber lines is shown in FIG. 1B. In FIG. 1B, a Remote Measurement Unit (RMU) is utilized. In this system, an RMU conducts all testing at the remote end. The remote terminal of the digital loop carrier includes a test bus, and a talk and control path which are coupled to the RMU. The RMU is controlled by the LMOS via a dial-up connection. Although the DC bypass pair is eliminated in this embodiment, this system is rather slow, requiring a proprietary LMOS interface, and only allows for POTS testing.
A third alternative is the use of metallic channel emulation equipment, coupled at the central office to similar emulation equipment situated at the remote terminals. This equipment emulates a metallic pair connection between the test systems coupled to bypass pairs in the central office and corresponding bypass pairs at remote terminal over the DLC. One such emulation unit is the Tollgrade MCU 4496 metallic channel unit manufactured by Tollgrade Communications, Inc., Wilmington, Del. The essential principles of the metallic channel unit are described in U.S. Pat. No. 5,457,743 entitled "Metallic Channel Unit Network," Inventor Frederick Kiko, which is a continuation-in-part of U.S. Pat. No. 5,202,919, entitled "Metallic Channel Unit Network," inventor Frederick J. Kiko.
A block level representation of this test system architecture is shown in FIG. 1C. As shown therein, the MCU unit is coupled to the PGTC by the Channel Test Unit (CTU) which gives Tip and Ring connections to the metallic channel unit. A corresponding MCU is provided at the remote end of the digital loop carrier and the Tip and Ring connections at the remote end of the system are emulated at the central office end. Hence, the emulated wire pair connection causes a load appearing at the remote end of the system to be connected via a wire pair to the central office end of the system and vice versa.
One disadvantage of the metallic channel unit is that it is specific to the particular digital loop carrier which is being utilized. Each type of DLC has its own proprietary interface for the metallic channel unit. This requires the telephone company to maintain an inventory of multiple types of MCU's. For example, in the MCU 4496 product information sheet (Issue 3B, list 1), the version of the MCU for the AT&T SLC96 DLC is described. In each case, the MCU unit must be integrated into the COT as well as the RT, as illustrated in FIG. 1C. Another disadvantage is that the MCU requires two DS0 digital channels to link the CO end with the remote end.
Another difficulty with the MCU is in the metallic emulation function. FIG. 2 is a reproduction of FIG. 2 of U.S. Pat. No. 5,457,743 (the '743 patent) which shows an equivalent circuit of the MCU. As shown in that patent, the two ends of the tip line connection are essentially identical in their implementation. The practical difficulty in implementing this system is in balancing the opposite sides of the circuit to make the system appear as a piece of cable with very high DC impedance to ground. This requires extremely accurate gain matching on both sides of the digital loop carrier (the remote end and the central office end). The '743 patent solution therefore requires great precision or careful adjustment of each unit in order for the emulation function system to work properly.
A further disadvantage of the MCU is that it is limited to testing of POTS systems; no provision for testing of special services is made.
As the need exists to test special service lines also, alternatives for meeting this need have been developed. An alternative for testing special services connections is the special services Remote Test Unit (RTU) shown in FIG. 3. A separate RTU and Test System Controller (TSC) are provided for the special services line at the remote end of the system. However, such systems are commercially unattractive because an RTU/TSC must be placed at each remote site where special services are to be implemented. This requires additional cost, and space at the remote terminal site, and the test equipment may be limited to testing special services only.